An attempt has been made to apply a metal oxynitride in which oxide ions (O2−) and nitride ions (N3−) coexist in the crystal with a photocatalyst, a pigment, a phosphor, a giant magnetostrictive material, a dielectric substance and the like (NPL 1).
A metal oxynitride is generally produced by heat-treating mixed powder of oxides as the raw materials in an ammonia flow at a high temperature of 800° C. or higher for several days. By the method, oxynitrides such as ATaO2N (A is Ca, Sr or Ba) and PrTaON2 are obtained, and the oxynitrides are expected to be applied as photocatalysts (NPLs 2 and 3).
A perovskite metal oxynitride represented by ABO3-nNn (A is a cation such as Li+, Ba2+ and Sr2+, and B is a metal cation such as Ti4+, Zr4+ and Sn4+) is used as a dielectric substance (PTL 1). PTL 1 describes a method for producing BaTaO2N, SrTaO2N, BaNbO2N, LaTaO2N, LaTiO2N, NdTaON2, SmTaON2 or the like by nitriding a raw material oxide in an atmosphere of ammonia gas at 900 to 1000° C. using an electric furnace.
Moreover, a perovskite metal oxynitride represented by AB(O, N)3 (A is Ca, Sr, Ba, La, Pr, Nd, Sm, Eu or Ce, and B is W, Mo, V, Nb, Ta or Ti) is used as an electric conductor (PTL 2). PTL 2 describes a method in which the metal oxynitride is produced by nitriding a composite oxide of metals A and B at 700 to 900° C. in an ammonia flow.
Furthermore, a method for producing an electron emission material containing an oxynitride perovskite containing a first component containing at least one of Ba, Sr and Ca and a second component containing at least one of Ta, Zr, Nb, Ti and Hf as metal element components, wherein the raw material composition is thermally processed in a nitrogen gas-containing atmosphere at 800 to 2000° C. with carbon placed near the raw material composition, is also known (PTL 3).
A piezoelectric material containing a tetragonal perovskite oxynitride containing Bi and a tetravalent element (PTL 4) and a piezoelectric material containing a tetragonal perovskite oxynitride containing Ti, Nb and Ta (PTL 5) are also known. These oxynitrides are produced by a method in which mixed powder of raw materials thermally processed at a high temperature or by a film-formation method such as sputtering.
These perovskite oxynitrides are generally obtained by a method in which a mixture of metal oxides is heated in an ammonia gas atmosphere to a high temperature and burned or a method in which a perovskite oxide is nitrided with high-temperature ammonia gas. An example in which barium titanate has been nitrided by the high-temperature ammonia gas nitriding method has been reported (NPL 4).
With respect to perovskite hydrides, cobalt oxide-hydride such as LaSrCoO3H0.7 or Sr3Co2O4.33H0.84 has been reported (NPLs 5 and 6). The present inventors have reported the synthesis of titanate oxyhydrides represented by the formula ATi(O, H)3 (A is Ca2+, Sr2+ or Ba2+) as a basis (NPLs 7 to 9 and PTL 6). The oxyhydrides are compounds in which hydrogen coexists as hydride (H−) with oxide ions (O2−) and are prepared by reducing a precursor ATiO3 with a metal hydride such as CaH2, LiH or NaH topochemically. The oxyhydrides are characterized by having hydride ion/electron mixed conductivity and hydrogen absorption/desorption properties.